A 20-year experiment showed that the repeated cloning of the same mouse reached the fifty-eighth generation, which represents the maximum limit for this replication.
It is worth noting that asexual reproduction in mammals is unstable in the long term, as genetic mutations accumulate rapidly in cloned offspring. The maximum number of consecutive clones appears to apply to all vertebrates.
Michael Lynch, an evolutionary biologist at Arizona State University, said: "In animal breeding, cloning is a way to maintain the optimal genome were it not for the problem of mutations. Once a mutation appears in a breed, it stays there permanently."
The experiment was conducted by reproductive biologist Teruhiko Wakayama of Yamanashi University in Kofu, Japan, who obtained the first cloned mouse in 1997. In the cloning, he used the nuclei of cells from living mice, and others from dead mice that had been preserved between 3 and 16 years, in addition to freeze-dried cells, cells taken from urine and feces, and freeze-dried sperm that had been kept on the International Space Station for about six years.
The experiment began in January 2005, and by 2013, 25 generations of clones had been produced. However, the cloning process became increasingly difficult after the 27th generation, and by the 58th generation, viable individuals were no longer being produced. This was attributed to the accumulation of serious mutations affecting the DNA sequence, with the rate of these mutations exceeding the normal rate in the clones by three times. Significant chromosomal changes were also observed, including deletions, inversions, rearrangements, and the loss of one of the X chromosomes.
Atsuo Ogura, a reproductive biologist at the RIKEN Center in Tsukuba, described these findings as the first clear experimental evidence that the accumulation of mutations in asexual reproduction can ultimately lead to the collapse of a breed. He cautioned, "To preserve valuable animal breeds, it is essential to store a large number of somatic cells in advance and avoid repeated serial cloning."
